The present invention relates to light bulbs generally used in stores, exhibition halls and the like.
The conventional light bulbs, such as halogen bulbs, generally contain therein a getter for suppressing a water cycle effect which is responsible for shortened service life of the bulbs (see Japanese Examined Patent Publication No.57-1862 (1982) and Japanese Unexamined Patent Publication nods.8-129994 and 8-1299905(1996)).
As shown in FIG. 6, a light bulb disclosed in Japanese Unexamined Patent Publication No.8-129994, for example, includes a quartz glass envelope 18 which is formed with a sealing portion 17 at one end thereof and contains therein a pair of lead-in wires 19, 20 extended externally of the sealing portion 17, a tungsten filament 21 suspended between the lead-in wires 19, 20 along an envelope axis, and a quartz glass stem 22 interposed between the filament 21 and the sealing portion 17 and supporting the pair of lead-in wires 19, 20.
The quartz glass envelope 18 is filled with predetermined amounts of a gas mixture of argon and nitrogen and of hydrogen bromide.
A tantalum getter 23 is supported at a place closer to the filament 21 than to the quartz glass stem 22, or at an upper part of the lead-in wire 20.
Since a conventional halogen bulb like this has the tantalum getter 23 disposed closer to the filament 21 than to the quartz glass stem 22, tantalum oxides liberated from the tantalum getter 23 reduced in mechanical strength due to its reaction with oxygen are carried to the vicinity of the filament 21 by convection currents occurring within the quartz glass envelope 18. Subsequently, the liberated tantalum oxides floating near the filament 21 are thermally decomposed so that tantalum is deposited on the filament surface 21 to form thereon an alloy of tantalum and tungsten (hereinafter, simply referred to as alloy). Since this alloy has a melting point (2,850xc2x0 C.) lower than that (3,400xc2x0 C.) of tungsten, a lower melting point portion is formed on a part of the filament 21. The lower melting point portion evaporates excessively to cause the filament 21 to break. Thus, the bulb life is shortened.
The invention contemplates a solution to this problem and has an object to provide a light bulb which can prevent a filament coil from breaking, thereby securing extended bulb life.
A halogen bulb according to the invention comprises a glass bulb having a sealing portion at one end thereof and containing a pair of lead-in wires extended externally of the sealing portion, a filament coil suspended between the lead-in wires, and a stem interposed between the filament coil and the sealing portion and supporting the lead-in wires, wherein a getter is located in an exposed manner between the sealing portion and the stem.
In this arrangement, the stem hinders convection currents occurring around the filament coil from carrying tantalum oxides liberated from the getter to and around the filament coil. Accordingly, an alloy of tantalum forming the getter and tungsten forming the filament coil can be prevented from forming on the surface of the filament coil and hence, the filament coil can be prevented from breaking. As a result, a light bulb having an extended service life can be provided.